This invention relates to crosslinkable blends of low viscosity short arm star polymers of conjugated dienes and, optionally, vinyl aromatic hydrocarbons. More particularly, the invention relates to functionalized crosslinkable blends of low viscosity short arm star polymers, especially hydroxy and epoxy functionalized and crosslinkable low viscosity short arm star polymers.
Block copolymers can be obtained by anionic polymerization of a conjugated diene, optionally with a vinyl aromatic hydrocarbon, by using an organic alkali metal initiator. One commonly used method involves first polymerizing monomer A, such as styrene, to form a living polymer block. Then monomer B, such as butadiene or isoprene is polymerized at the end of the A block to form an A-B block polymer which is still living. This A-B block polymer is referred to as a diblock polymer or, when discussed in the context of a branched or star polymer, as a block polymer arm or living polymer arm. These block polymer arms; are then coupled together by the addition of a coupling agent which reacts with two or more of the arms to form one compound. If two arms react, then the polymer will have the structure A-B-X-B-A where X is the coupling agent. Since X is only one molecule and is relatively small, the polymer looks and acts like a linear A-B-A block copolymer. This process is generally described in U.S. Pat. Nos. 3,595,941, 3,468,972 and 4,096,203.
It is well known that the viscosity of such linear polymers can be lowered by adding solvents or other low molecular weight organic liquids. There are advantages in many applications such as paint and coatings to having a lower viscosity for the polymer. The amount of volatile organic compounds (VOC's) must be limited in most coatings and adhesives. Therefore, it is highly desirable to achieve viscosity reduction with minimal added VOC's. Unfortunately, the presence of reactive diluents, etc. also dramatically decreases the overall molecular weight of the polymer blend. A reduction in molecular weight is particularly undesirable when the intent is to produce a crosslinkable polymer composition. A higher molecular weight polymer requires fewer reactions to cure or crosslink. Higher molecular weight polymers have a smaller number of molecules per unit mass and this means that there are more reactive double bonds per molecule per unit mass which are available to participate in the crosslinking reaction. Further, if large amounts of solvents or reactive diluents have to be added to decrease the viscosity, the cost goes up the VOC increases and possibly the efficiency of the radiation crosslinking may be adversely affected by the solvent.
Star polymers are also crosslinked by exposure to radiation. They are produced by a coupling process. The coupling agents used are polyfunctional coupling agents or monomers. Coupling agents such as divinylbenzene, may polymerize or oligomerize as well as react with living chain ends. The results of these two types of reactions is the formation of a star shaped polymer having up to 40 or 50 arms attached to a central core which is composed principally of the coupling agent.
Star polymers generally have relatively high molecular weights because of the number of polymer arms present in the molecule. Because of the number of polymer arms, there are quite a number of potential crosslinking sites in the molecule as well. It would be advantageous to provide a crosslinkable star polymer composition with a relatively low viscosity which was able to maintain its relatively high molecular weight without the addition of large amounts of solvent or co-reactive diluents. The present invention provides such a polymer and functionalized versions thereof.